Pattern catching in a photoelectric pattern contour tracing system

ABSTRACT

A pattern contour-tracing system which is capable of intercepting and locking on the pattern contour from a position removed from the contour without the use of a leadin contour. The pattern contour-sensing head is pointed so as to travel in a line toward the contour with the sensing head steering means locked out until the sensing means detects that the sensing is within a predetermined distance of the contour. The steering means is then unlocked so that the sensing head can be automatically steered by the servo means to make a smooth transition into the path defined by the pattern contour. A photosensitive tracing system is disclosed which is operable to lead into and trace the pattern contour whether the contour is defined by a line or an edge.

United States Patent [72] Inventors Francis G. Bardwell Elmhurst;Richard A. Mazur, Chicago, Ill. [21] Appl. No. 819,787 [22] Filed Apr.28, 1969 [45] Patented Feb. 23, 1971 [73] Assignee Stewart-WarnerCorporation Chicago, Ill.

[54] PATTERN CATCHING IN A PHOTOELECTRIC PATTERN CONTOUR TRACING SYSTEM21 Claims, 10 Drawing Figs. [52] US. Cl 250/202, 250/219, 318/18 [51]Int. Cl..; G0ln 21/30; G05b 1/00 [50] Field of Search 250/202, 219, 206;318/18; 340/1463; 178/6, 7.6 [56] References Cited UNITED STATES PATENTS2,999,938 9/ 1961 l -lann et a1, 2501202 Primary Examiner-John KominskiAssistant Examiner-E. R. LaRoche Attorneys-Augustus G. Douvas, William JNewman and Norton Lesser ABSTRACT: A pattern contour-tracing systemwhich is capable of intercepting and locking on the pattern contour froma position removed from the contour without the use of a leadin contour.The pattern contour-sensing head is pointed so as to travel in a linetoward the contour with the sensing head steering means locked out untilthe sensing means detects that the sensing is within a predetermineddistance of the contour. The steering means is then unlocked so that thesensing head can be automatically steered by the servo means to make asmooth transition into the path defined by the pattern contour. Aphotosensitive tracing system is disclosed which is operable to leadinto and trace the pattern contour whether the contour is defined by aline or an edge.

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M5110 4-8 LINE s I? CELL /56 PATTERN CATCHING IN A PHOTOELECTRIC PATTERNCONTOUR TRACING SYSTEM BACKGROUND OF THE INVENTION 3,213,282 and3,286,142, all owned by the common assignee.

Stewart-Warner Corporation. By the very nature of contour tracingsystems, the leadin problemis an inherent one which has heretofore notbeen easily corrected.

The most widely used tracing systems of today have a sensing means suchas one or more photocells, at least one of which is positioned so as toview a portion of the tracing pattern slightly forward of thesteeringaxis of the rotation of the sensing head. This is required togive the system proper steering sense in the same manner and for thesame purposes that a motor vehicle operator trains his eyes on the roadat a substantial distance in front of the vehicle rather than at a spotclose to the steering axis of the frontwheels. When the sensing meansindicates a positional deviation from the contour steering means itcauses the sensing head to rotate the sensing means towards the contour.In this manner the sensing head traces along the contour, and thesteering axis to which the work tool is fixed follows closely behind tocut the contour from the workpiece in accordance with the tracingpattern.

During leadin procedures, however, when the tracing head is approachingthe contour from a position off of the contour, the sensing means willeventually sense the contour and, because of its nature, give anindication to the steering means to turn the head towards the contour.If the angle of approach to the contour is substantial and the feed rateof the tracing head is great enough with respect to the speed ofrotation of the head, the head can proceed completely across the patternwithout ever locking in on it. if the tracing system has an off patterncontrol as in the system disclosed herein, the tracer may come to acomplete stop on the other side of the pattern from which it approached.If the sensing head of the tracer approaches the contour at a somewhatlesser angle, it might overshoot the pattern by a small amount before itlocks onto it. This is, the tracing head would cross the line slightlyand then reverse directions so as to lock onto the pattern and thuscause the work tool attached thereto to .distort the pattern cut fromthe workpiece. Some of the prior art systems could be made to lead intothe contour properly, but only if the tracing head was steered onto thepattern at a very slight angle with the contour thereto. Because ofthese problems, machine operators usually have used manual procedures tosteer the tracing head onto the pattern. The operator causes the head toapproach the pattern at an angle of perhaps or less with respect to thecontour and then manually holds the tracing head from turning by meansof a steering wheel until the sensing head catches the pattern, at whichtime he releases'his hold of the wheel and permits the automaticsteering control to guide the tracing head.

SUMMARY OF THE INVENTION The present invention permits the automaticleadin of the tracing head onto the pattern contour without the aid ofleadin contours on the pattern and without the manual control of theoperator. The system enables the tracing head to approach and lock ontothe pattern contour in a positive manner without losing or overshootingthe contour. Included in the system are control means for catching thepattern which comprises means for pointing the front-to-back axis of thetracing head toward the contour and means including the tracer drivingmeans for driving the tracer in the direction of the frontto-back axistoward the contour. Means are provided for detecting when the sensinghead reaches a position which is a predetermined distance from thecontour, and means responsive to the detecting means are utilized toprevent the operation of the automatic tracer head steering means untilthe sensing head reaches the predetermined distance position. If thetracing system is of the optical type in which sensing head position isdetermined by the electric response of the photosensitive means, thepredetermined distance at which the tracer and steering means are madeoperable is determined by the magnitude of the electric response of thephotosensitive means. In the preferred embodiment shown and describedherein, the system is adaptable to be operable whether a line or an edgeforms the pattern contour. In the case of line tracing, however, specialprovisions are made to eliminate ambiguities which could cause thesteering means to be enabled too soon as it approaches the line, as willbecome more clear from the following description of the line-tracingembodiment.

It is to be understood that although optical tracers are discussedherein the broadest aspects of the invention can be applied toother'types of tracers including mechanical feeler types, cathode raysensor types, etc.

This invention will be better understood by a further reading of thefollowing description, especially when taken in line with theaccompanying drawings in which:

FIG. 1 is a block diagram of a line and edge-tracing system embodyingthe pattem-catching techniques of this invention;

FIG. 2a is a diagrammatic representation of a line tracer approachingthe line;

FIG. 2b is a graphical representation of the signal amplitude variationsas the tracer of FIG. 2a approaches the line;

FIG. 2c is a graphical representation of the logic functions performedby the system as the tracer of FIG. 2a approaches the line;

FIG. 3a is a diagrammatic representation of an edge tracer as itapproaches an edge;

FIG. 3b is a graphical representation of the signal amplitudes as thetracer of FIG. 3a approaches the edge;

FIG. 30 is a graphical representation of the logic functions of thesystem as the tracer of FIG. 3a approaches the edge; and

FIGS. 4 through 6 are schematic diagrams of the line and edge tracingsystem shown in block diagram in FIG. I.

The system shown in block diagram in FIG. 1 indicates the basicfunctions of the invention and the means for performing same as they mayapply to one of the more simple patterntracing systems. The basictracing system is of a type such as disclosed in the aforementionedRedman U.S. Pat. No. 3,286,142 which includes a pattern-sensing head 10,steering means 12 for rotating the sensing head 10 to cause it to followthe path defined by the pattern contour, and driving means 14 forcausing the sensing head to advance along the pattern contour.

The sensing head 10 comprises means for sensing the pattern contour suchas one or more photocells 16. Although the preferred embodiment of theinvention disclosed herein is directed to optical tracers havingphotocells, it is to be understood that the broadest aspects of thisinvention are applicable to other types of tracers including cathode raytubescanning tracers and mechanical feeler tracers. The scanning head ofthe aforementioned Redman patent includes single photocell 16 whichviews the pattern contour and develops electric signals in accordancewith deviations therefrom in a well-known manner which will be morefully explained with respect to FIGS. 20 and 3a. The Redman patentsystem is capable of following either a thin line or an edge asdetermined by the operational mode of a line-edge selector 18. Thephotocell signals are amplified by a preamplifier 20 and provided to thesensing head steering means which serves to rotate the sensing head inaccordance with the photocell signals. The signal from the sensing headis passed through a gated amplifier 26 and is amplified by a driveamplifier 22 which controls the operation of a steering motor 24 torotate the sensing head 10. The gated amplifier 26 in the steering means12 is provided to permit the passage of the sensing head signals to thedrive-steering amplifier only under the desired condition when thesensing head is within a predetermined close proximity of the patterncontour, as will be described hereinafter.

The steering motor 24 is coupled mechanically or electrically to thedriving means 14 so as to cause the sensing head to be driven in theproper direction along the pattern contour. in the case of the system ofthe aforementioned Redman patent, the driving means is geared to thesteering means for rotation thereby and includes a traction wheel (notshown) which is driven by a feed motor 28. The drive motor is energizedby feed motor drive circuit 30 which may include manual means forselecting the desired feed speed of the tracer about the contour. It isto be understood, however, that other drive means may be used such as acoordinate drive system in which two motors operate independently todrive the sensing head in respective directions responsive to properlyresolved signals to follow along the pattern contour.

The Redman system also includes an off-pattern detection system whichcauses the tracer'to' stop its motion whenever the sensing head straysoff the pattern contour. Such a system includes an off-pattern detectioncircuit 32 and an off-pattern relay and driver circuit 34 which operateto open the circuit between the feed motor 28 and the feed motor drivecircuit 30 when the signal at the output of the sensing headpreamplifier indicates that the sensing head is not viewing a segment ofthe pattern contour. The functional components between the off-patterndetection circuit 32 and the off-pattern relay driver circuit 34 as wellas the remaining functional components in FlG. 1 not heretoforementioned are utilized to complete the pattern catching control systemfor enabling the tracer to lead into the pattern contour in accordancewith this invention and will be described in more detail hereinafter.

Reference is now made to FIGS. 2a, 2b and 2c for a better understandingof the operating principles of this invention, especially as it appliesto an optical-type tracer for following along a thin line such asdisclosed in the aforementioned Brouwer and Redman patents. Asschematically shown in FlG. 2a, the sensing head 10 comprises aphotocellas represented by its photosensitive area 16a which is opticallypositioned to view a portion of the pattern slightly in advance of therotational axis 38 of the sensing head 10. An opaque shade 40 is causedto oscillate between the sensitive area 16a and the pattern image in thedirection indicated by the arrowed line referenced d. The shutter 40travelsto the limits indicated by the reference line d on either side ofthe center of scan which normally defines the front-to-back axis 39 ofthe sensing head at a carrier frequency of, for example, 60 hertz.

The steering means of the tracer system operates in response to thefundamental component signal (60 hertz) to rotate the tracer-sensinghead about its axis 38 Thus, it a portion of the line is viewed by thesensitive area 16a to the right of the front-to-back axis 39, thesteering means will cause the sensing head 10 to rotate in a clockwisedirection about the axis 38. On the other hand, if the line is viewed tothe left of the front-tmback axis 39, the sensing head 10 is ordinarilycaused to rotate in a counterclockwise direction. In this manner therotational axis 38 will be caused to travel essentially down the centerof the pattern line 40 to accurately trace the desired contour.

The graphs of FIG. 2b are representative of the amplitudes of thefundamental reference frequency (60 hertz) and the second harmonic ofthe reference frequency (120 hertz) in the photocell output signal asthe tracing head travels in the direction of its front-to-back axis 39towards the thin line 42 representing the pattern contour. As may beseen, when the tracing head is positioned such that an image of the lineis not projected on the sensitive area 16a, as indicated by the centerof the photosensitive area at position 1, there is no 60 hertz or hertzsignal present in thephotocell output. However, as the sensitive area16a proceeds along the path indicated by the dotted line 44, both a 60hertz component and a 120 hertz component will begin to be generatedwhen the sensitive area encounters the pattern line 42 at its right side46. As it proceeds further along the path 44 so that more of the line 42is projected onto the sensitive area 16 a, the amplitude of thefundamental and second harmonic signals continue to increase with thel20 hertz signal reaching a maximum when the center of scan ispositioned over the center of the line 42. The 60 hertz component,however, reaches a maximum and then falls to zero as the sensing headapproaches the center of the line 42, in the manner described more fullyin the aforementioned patents. As is well known, .there is a phase shiftin the fundamental frequency component as the tracing head passes fromone side of the center of the line 42 to the other, and it is this phaseshift which is used to give the tracing system directional sense.

FIG. 2b also includes a curve representing the additive result of theabsolute amplitudes of the l20 hertz and the 60 hertz signals. Thissignal is used by the off-pattern detector 32 (FlG. l) to determine whenthe tracer is straying off the line. That is, as long as the absoluteamplitude of .the 60 hertz plus 120 hertz signal is above the off-linethreshold level in H6. 2b as the tracer is following a line, theoff-line circuits will maintain the driving means in operation. If thetracer strays from the line such that the 60 hertz plus l20-hertz signallevel falls below the off-line threshold level, the driving means willbe caused to stop. v

The problem to which this invention is directed occurs when an operatoris trying to direct the tracer to the pattern contour from a positionoff the contour at the beginning of the tracing operation. When thesensing head 10 is approaching the line 42 in the manner shown in H6. 2aalong the path 44, the sensitive area 16a will first encounter an imageof the pattern line 42 at its right side 46. As previously stated, thiswill cause the steering means to 'rotate'the sensing head 10 in aclockwise direction. Thus, the sensing head has a tendency to followalong the path indicated by the dotted line 48 toward the pattern line,and under certain circumstances might even cross the line completely andend up on the right side of the line 42 with the photocell signalindicating to the offline circuit that the tracer driving means shouldbe shut down. The strength of this tendency to cross the line is adirect function of the angle alpha between the line of approach 44 andthe pattern line 42. in previous systems the tracer was prevented fromcrossing the line by the operator manually holding the sensing head toprevent rotation until the head was on the line.

The system of the present invention overcomes this problem by preventingthe steering means from turning the sensing head 10 as it approaches theline 42 along the path 44 until the system detects that the center ofthe sensitive area 16a is at a position very close to the center of theline 42 as might be represented by point 6 along the path 44 in FIG. 2a.it detects the position 6 as the line tracer approaches the pattern line42 by detecting the point at which the absolute amplitude of thefundamental 60 hertz signal falls below a predetermined threshold valueindicated B at the corresponding point 6 on F [6. 2b. At that point thesteering means is gated into action and the tracing head will then takea smooth transitional path onto the line 42.

It will be noted in FIG. 2b that the absolute amplitude of the 60 hertzsignal will also be below the B, threshold level as long as the centerof the photosensitive means has not yet reached the point referenced 2on the path 44 to the pattern line 42. Special care must therefore betaken to maintain the steering means deactivated until the B2 amplitudethreshold condition is reached from the condition which provides agreater absolute amplitude of the 60 hertz signal. The off-linethreshold and the B1 threshold are used to detect this condition in amanner to be hereinafter described.

. position of the sensing head 10.

At the start of operation when the tracing head is at an at rest modeand the scanning head is positioned so that the photosensitive means 16is not viewing the pattern line, the off-pattern detector 32 does notreceive a 60 hertz or a 120 hertz signal from the output of thepreamplifier 20. Thus, the off-pattern detector maintains a triggerfunction circuit 60 in a condition such that its A output is zero. Andgate 52 is therefore closed and no signal emanates therefrom through ORgate 54 to the off-pattern relay and driver 34. The A functionrepresents the status of the off-pattern detector in that a patterncontour is being viewed when the A function is in its 1 state and thecontour is not being viewed when the A function is in its zero state.

The F input to the OR gate 54 is also at a zero condition before thestart of operations so that the off-pattern relay and driver 34maintains the feed motor 28 deenergized. To drive the sensing head tothe line it is desired that the driving means 14 operate in spite of thefact that the sensing head is not viewing the line and this isaccomplished by the operator physically pointing the sensing headtowards the line and actuating the start switch 56. The momentaryclosure of switch 56 sets a flip-flop 58 so that its F output changes toa 1 condition which is fed through the OR gate 54 to the Off-patternrelay driver circuit 34 causing the energization of the feed motor 28 bythe closure of relay contact 340. The steering means 12 is preventedfrom rotating the sensing head 10 at this time, how ever, because theAND gate 52 is kept closed because of the zero state of the A output oftrigger function 50 and the zero state of the Foutput of the flip-flop58. The AND gate 52 controls the gate 26 in the steering means and willmaintain that gate closed until the simultaneous status of the detectionof the pattern line by the sensing head represented by the 1 state ofthe A output of trigger function 50 and the arrival of the sensing headat the position 6 (FIGS, 2a, 2b) which, as will be seen later, isrepresented by a 1 state at the I output of the flip-flop 58. Themomentary closure of switch 56 also resets a memory cell 70 (H6. 1) toprovide a zero condition at its 0 output, the purpose of which will bebetter understood later. The position at which the operation is startedby the operators actuation of switch 56 is indicated as point 1 in therepresentations of FIGS. 2a, 2b and 2c.

As the sensing head it) proceeds along the path 44 towards the patternline 42, the photosensitive means will encounter a portion of the lineat its right side 46 and the amplitude of the 60 hertz, the 120 hertzand the 60 hertz plus 120 hertz signals will start to increase asindicated in FIG. 2b. When the sensing head reaches the position 2 onthe path 44, the 60 hertz signal reaches the B threshold level. The Blevel is detected by the logic circuits in FIG. 1 wherein thefundamental or 60 hertz component of the scanning headsignal is passedthrough a filter 62 (FIG. 1) and an amplifier 64 to the input of an inverter circuit 66. As will be seen in the detailed description of thecircuit 66, it operates to feed a 1 condition to an input of AND gate 68when the level of the 60 hertz signal is below the B threshold level anda zero condition when it is above the B level. The gate 58 controls theresetting of flip-flop 58 which in turn controls the normal operation ofthe tracer steering means and the driving means. Thus, the zerocondition of the 52 function will maintain the gate 68 closed as long asthe 60 hertz signal is above the B level. The AND gate 68 is kept closedduring the period before the 3; level is reached by a zero condition atthe Q output of the memory cell 70 to the AND gate 63, thus providing ablocking function during this ambiguous portion of the cycle.

The progress of the sensing head along the path 44 beyond point 2 causesthe signal levels to further increase. When the position 3 is reached asufficient portion of the line 42 is scanned by the photosensitive meansto provide a 60 hertz plus hertz signal of the off-line thresholdamplitude. At this point the off-line detector 32 causes the triggerfunction circuit 50 to switch to its 1 condition at the A output, butthis has no effect on the operation of the AND gate 52 because of thezero state of theFline from the flip-flop S8. The I state of the triggerA function does however prepare one input to the AND gate 68 and aninput to a gated AND circuit 60 through a set of switch contacts 18aassociated with the line-edge selector 18 in the sensing head 10. Theother input of the gated AND 60 receives the 60 hertz component of thesensing head signal from the amplifier 64. The gated AND circuit 60 isadjusted to open only when the amplitude of the signal from theamplifier 64 reaches the B1 threshold level indicated in FIG. 2bprovided, of course, that the online trigger A function signal isreceived.

As the tracer proceeds further a long the path 44 to the point 4, the B1threshold level is reached which opens the gated AND circuit 60 toprovide a set signal to the memory cell 70. The Q output of the memorycell 70 goes to its 1 condition removing the blocking function at theinput to the AND gate 68. Although the A function of the trigger circuit50 is also in the 1 condition, the ate 68 is now inhibited because ofthe zero condition of the function atits third input.

The gate 68 will remain inhibited and no further logic functions willoccur until the amplitude of the 60 hertz signal falls past the BIthreshold, which occurs at position 5 of the FIG. 2 representations, tothe B2 threshold level which occurs at position 6. At that time the gate68opens and resets flip-flop 58. The F output thereof goes to a 1condition opening AND gate 52 which in turn opens the gated amplifier 26in the steering means 12 to permit the steering motor 24 to be energizedin accordance with the sensing head output signal. The F output of theflip-flop 58 which controls the operation of the offpattern relay anddriver 34 through the OR gate 54 goes to its zero condition, but thecircuit 34 still receives an enabling signal from the output of the ANDgate 52 through the OR gate 54. Thus, the off-pattern relay contacts 34awill remain closed to energize the feed motor 28 and will stay that wayunless and until the off-pattern detector 32 indicates that the sensinghead has lost the pattern line.

The sequence of the logic functions of the system operating in theline-tracing mode are shown in the representations of FIG. 20 and arerelated to the various positions of the sensing head as it moves alongthe path 44 by the referenced numbers l-6. It may be seen that the logicfunctions are organized such that the line-catching system will operateover a wide range of settings of the off-line threshold level. That is,the system will properly operate whether the off-line threshold isabove, below, or between the B1 and B2 levels as long as the B2 level isbelow the BI level. The off-line A function does make the system fullyinsensitive to noise such as would be caused by random dirt spots on thepattern, provided, however, that the off-line threshold is above themaximum noise level one can expect from such dirt spots. Therefore, theoffline A function operates to inhibit the passage of any signals fromthe amplifier 64 to the memory cell 70 prior to the recognition of theline.

As previously indicated, the pattern-catching techniques of thisinvention are also applicable to an edge-tracing system. Therepresentations in FIGS. 3a, 3b and 3c will help to understand thefunctional operations of the edge tracer system to follow a smoothtransitional path onto the edge. The particular-type edge tracerrepresented herein is one such as shown in the aforementioned Redmanpatent which is convertible from a line tracer. It uses the samesensitive area 16a which is positioned forward of the rotational axis 38in the direction of the front-to-back axis 39, but the vibrating shutteris not used. Instead a pulsating light source 72 is used whichilluminates the pattern at the reference frequency of, for example, 60light pulses per second.

In this type of an edge tracer the signal at the output of the photocellcomprises the fundamental component but no second harmonic. The 60 hertzsignal goes from a maximum amplitude state when the photocell is viewingonly the white side of the edge to a minimum or zero amplitude when thephotocell is viewing only the black side of the edge. To give the systemdirection sense the photocell output is mixed with a 60 hertz referencesignal which is approximately half the amplitude of the maximum signalwhen the photocell is viewing the white side of the pattern with itsphase shifted 180 from the photocell signal. The resultant is a 60 hertzsignal the amplitude and phase of which are indicative of the amount anddirection of deviation of the center of the photocell from the image ofthe pattern edge. The edge-catching control operates on the absoluteamplitude of the 60 hertz signal made up of the photocell signal plusthe reference signal so that it is operable whether the tracer isapproaching the edge from the black portion of the pattern or the whiteportion.

As in the case of the line tracer, the steering means 12 is maintainedinoperable as the tracer approaches the edge until the absoluteamplitude of the 60 hertz signal drops below the B2 threshold. Incontrast to the line-tracing situation, however, there is no ambiguouscondition since the absolute amplitude of the 60 hertz signal is alwaysabove the B2 threshold except for when the sensing head is within thevery narrow portion of the pattern on either side of the pattern edge.This, in the edge tracing mode only the B2 signal is relevant to thelogic operation of the patterncatching system. The ofi'-line A functionand the B1 threshold are not required, but as will be seen, they must beaccommodated in the embodiment shown because of its operation as a lineor edge system.

Referring again to FIG. 1, when the system is adapted for edge tracing,the selector 18 is switched to its edge-tracing mode with the switchcontacts 18a residing in the edge position. As before, the tracer isstarted towards the edge from the position off the edge by a momentaryoperation of the start switch 56. The flip-flop 58 is set-so as todeactivate the steering means 12and activate the driving means 14 tocause the sensing head to be driven towards the edge in the directionmanually set by the operator. Tl i e reset of flip-flop 58 is controlledoniy by the status of the B2 signal in the edge mode in view of thepermanent A=1 condition provided through the switch contacts 18a toinputs of the AND gate 68 and the gated AND circuit 60. Since thesensing head signal is at a maximum level when the tracer is completelyoff of the edge the output of amplifier 64 exceeds the B1 level and thegate 60 insures that the memory cell 70 provides a high Q signal to thegate 68. With the two upper inputs of gate 68 in the high or :11 state,its output will be dependent only on the level of the B2 signal at itsthird input. As the tracer proceeds along the path 44 and starts to viewthe edge, the A function of the trigger circuit 50 goes to its 1 stateto prepare the AND gate 52. This is the condition at the positionreferenced 2 in FIGS. 30, b and c. As the tracer proceeds on, point 3 isreached which represents the position a't'which the amplitudt f the 60hertz signal falls below the B2 threshold level. The 132 inhibit isremoved from the gate 68 causing it to open and reset the flip-flop 58.As in the line-tracingcase, the reset of flip-flop 58 enables thesteering means 12 to permit the steering motor 24 to rotate the sensingmeans 10 in accordance with the sensing head signals, and permits theoff-pattern circuits to control the operation of the driving means 14.

Reference is now made to FIGS. 4, 5 and 6 for a detailed description ofthe schematic circuit of a preferred embodiment of the line oredge-catching control system The sensing head 10, with its photoelectricmeans 16, the lineedge selector l8 and preamplifier are not shown ordescribed in detail because they are adequately described in theaforementioned US. Pats. As indicated before, however, the photoelectricmeans 16 scans the pattern contour image at a reference frequency of 60hertz and the output of the preamplifier is a fluctuating signal whichincludes the reference frequency component, the amplitude and phase ofwhich indicate the amount and direction of transverse deviation of thephotosen sitive means from the pattern contour. It the case of linetracing the output of the preamplifier also includes a second harmoniccomponent of I20 hertz which is present only when the photosensitivemeans 16 is viewing at least a portion of the pattern line and may beused as an indication of when the sensing head is on or off the line.

The signal from the output of the sensing head preamplifier normallycontrols the operation of the steering means 12 which is made up of thegated amplifier 26, drive amplifier 22 and the steering motor 24. Thefluctuating signal on the output conductor 73 from the sensing head isapplied to the base of a transistor 74 which is connected as a commonemitter amplifier. The potentiometer 76 in the emitter circuit of thetransistor 74 serves as a gain control for the steering circuit in awell-known manner. An inductance 78 and capacitor 80 filter the outputfrom the amplifier 75 so that only the reference frequency compon'entor60 hertz signal is passed through capacitor 82 to the center contact 840of a manual steering control switch 84. The outer two contacts 84b and84c are connected to opposite phase components of a reference AC sourceso that by operator manipulation, a signal may be provided through theremainder of the circuit to cause the steering motor 24 to turn in onedirection or the other. When the switch 84 is in its center position,however, the steering circuit 12 can be operated responsive to thesignal from the sensing head and the amplified fluctuating signal isapplied to the base of a second. transistor 86 which is also connectedas a common emitter amplifier 87. The phase and amplitude-responsivefluctuating signal from the collector of transistor 86 is coupledthrough capacitor 88 to the primary 90 of a transformer 92, thesecondary 94 of which provides the input to the drive amplifier 22 forenergizing the steering motor 24. The steering motor 24 is operable inaccordance with the phase and amplitude of the'output of the driveamplifier 22 to steer the sensing head 10 with respect to the patterncontour in a well-known manner so that these circuits need not bedescribed in detail.

As previously mentioned, the amplifier 26 is a gated type, the gatingfunction being performed by a PNP transistor 96 which is operableresponsive to the circuits shown in FIGS. 5 and 6 to short out theprimary 90 during the leadin procedures of the tracer to the patterncontour at the beginning of the tracing operation. As indicated withrespect to the functional diagram in FIG. 1, the gated amplifier 26 iscontrolled in part by the status of the off-pattern detection circuits.These circuits are shown in FIG. 5 and operate responsive to the outputsignal of the sensing head preamplifier 20 on conductor 73, which in thecase of the line-tracing mode will contain a 60 hertz component and/or a120 hertz component, dependent upon whether the photosensitive means 16is centered on the line or is slightly off the line. if the pattern lineis not being viewed at all, there will be virtually no fluctuatingcomponent in the preamplifier output signal.

The sensing head signal from the output of the preamplifier 20 isapplied to the input of the off-pattern detector 32 at the base oftransistor 98. The transistor 93, connected as a common emitteramplifier 100, amplifies the sensing head signal and transmits itthrough coupling capacitor 102 to the base of a PNP transistor 104 alsoacting as a common emitter amplifier 106. The emitter circuit of theamplifier includes a set of contacts 18b forming a part of the line-edgeselector control 18 so that the gain of the amplifier may be selectivelycontrolled by means of potentiometers 1080, 10% to determine the properoperating points of the off-pattern circuits.

The transistor 104 conducts only on the negative swings of thefluctuating signal appearing on its base and charges a capacitor in itscollector circuit in accordance with the absolute amplitude of thefluctuating signal received from the scanning head. The voltage level ofthe capacitor 110 determines the state of the trigger function circuit50 which comprises transistors 112 and 114. The trigger circuit 50 issimilar to a Schmidt trigger except that in its quiescent condition bothtransistors 112 and 114 are nonconducting. That is, with no incomingsignal from the sensing head, the base of transistor 112 is essentiallyat ground voltage with its emitter at a positive voltage by the actionof the voltage-dividing resistors 116, 118 and 120 to maintain itnonconductive. The PNP transistor 114 therefore is also nonconducting inview of its direct connection to the collector of NPN transistor 112.The charge level at which the capacitor 110 switches the trigger circuit51) from one state to another is determined by the reverse bias providedby the voltage divider formed by resistors 116, 118 and 120. The sensinghead signal levels which are necessary to reach that capacitor chargelevel are represented as the offline and off-edge threshold levelsindicated in FIGS. 2b and 3b, and the position of the tracer withrespect to the pattern contour to cause the trigger circuit 50 to switchis determined by the setting of the gain control potentiometers 108a and108b in the amplifier 100.

Thus, in the case where the system is being used as a pattern linetracer, the trigger circuit 50 will be held in its state where bothtransistors 112 and 114 are not conducting as the tracer approaches theline until enough of the pattern line is viewed to cause capacitor 110to charge above the off-line threshold. Trigger function circuit 50 thenquickly flips to its conducting state to provide a high signal at thecollector of transistor 114 and hence through resistor 122 to the baseof transistor 124.

Of course, these circuits operate in the opposite mode when the traceris being used as an edge control. It will be recalled that the signalfrom the output of the preamplifier 20 in the sensing head is at maximumabsolute value when the photosensitive means 16 is not viewing a segmentof the pattern edge. Thus, the capacitor 110 is charged to a maximumvalue when the tracer is not viewing a portion of the edge and thetransistors 112, 114 of thetrigger circuit '50 are in their conductingstate. As the tracer approaches the edge, the charge on the capacitor110 will fall to a level at which the transistors 112, 114 will switchto their nonconducting condition to provide a low signal to the base oftransistor 124.

When the trigger circuit 50 is in' its conducting state to present apositive voltage to transistor 124, it also conducts to present a lowsignal on conductor 126 connected to its collector. This low is invertedby transistor 128 so that the signal at conductor 130 connected to thecollector of transistor 128 is high whenever the trigger functioncircuit 50 is in its conducting condition. When the tracer is in itsline-following mode, the signal on conductor 130 is passed through theline-edge switch contacts 180 to one input of the AND circuit 52 formedby diodes 132,, 134. This signal of course is high when the triggercircuit 50 is in its conducting condition indicating that the tracer isviewing the line and is low when the tracer is off the line.

It is to be noted that when the tracer is in the edge tracing mode thesignal delivered to the AND gate 52 is taken from conductor 126 becausethe trigger circuit 50 is in its nonconducting state when the tracer isviewing a portion of the edge and the input to diode 132 must bepositive when the tracer is on the edge or on the line. Under normaloperating conditions when the tracer is properly following the patterncontour, the other input to the AND gate 52 at the cathode of diode 134is also high so that a high is provided through the OR gate 54 formed byresistors 138, 140 and 142 to the base of transistor 136. The transistor136 forms part of the off-pattern relay and driver circuit 34 and servesto drive the off-pattem relay solenoid 144 in its collector circuit.Transistor 136 is conducting when the tracer is properly following thepattern contour so that the feed motor contacts 34a are closed to permitdriving of the tracer along the contour. The other input to the OR gate54 through resistor 143 permits driving the tracer towards the patterncontour even though a portion thereof is not being viewed by the sensinghead as previously described.

The pattern-catching logic function circuits shown in FIG. 6 utilize thesensing head output signal on conductor 73 and in the case of linetracing the signal status on conductor 130 from the collector oftransistor 128 in the oft-pattern circuit in FIG. 5. The conductor 130signals the status of the A function discussed with respect to FIG. 1,the 1 state of which indicates that the tracer is viewing the line.

The logic function circuits schematically shown in FIG. 6 perform thetask of maintaining the steering means 12 inoperable as the tracer isdriven towards the pattern contour until the tracer reaches apredetermined position closely adjacent the contour. At that point thesteering means is enabled to operate to steer the tracer onto thecontour and from there n operate in a normal tracing manner.

The status of the steering circuits is determined by the condition ofthe flip-flop 58 which may be seen comprises NPN transistors 146 and 148connected in a well-known manner. That is if the F function on conductor150 connected to the collector of transistor 148 is in its high or 1condition, the steering means may control the rotation of the sensinghead 10 through AND gate 52, provided of course, that the sensing headis viewing a portion of the contour pattern. The F function on conductor152 connected to the collector of transistor 146 controls the status ofthe off-pattern relay and driver circuit 34 through OR gate 54 to enabledriving of the sensing head towards the line as previously mentioned.

The initial status of the flip-flop 58 is controlled by the start switch56 which has the effec t of turning transistor 146 off and transistor148 on so that the F function on conductor 150 goes essentially toground and the F function on conductor 152 goes to approximately thesource voltage level. The high F function on conductor 152 istransmitted through the OR gate 54 (FIG. 5) to the base of transistor136 causing it to conduct through the relay 144 and thus cause the feedmotor to drive the tracer towards the pattern contounThe IowFfunction onconductor is applied to the cathode of diode 134 in the AND gate 52 andthe low output of the AND gate is transmitted via conductor 154 andthrough resistor 156 (FIG. 4) to the base of the PNP gating transistor96 in the steering circuit 12. Transistor 96 thus conducts and acts as ashort circuit across the transformer primary 90 to prevent steeringsignals from passing therethrough to the steering motor 24.

The flip-flop 58 will remain in this state until the remainder of thelogic function circuits of FIG. 6 determine that the tracer has reachedthe predetermined distance from the contour pattern, at which point thetracing means is enabled to steer the tracing head smoothly onto thepattern contour. To this end the sensing head output signal from theoutput of the preamplifier 20 is provided on conductor 73 and throughresistor 158 to the 60 hertz reference frequency pass filter 62 made upof inductors 160, 162 and capacitors 164, 166. The reference frequency60 hertz signal is applied to the base of transistor 168 which isconnected as a common collector amplifier. Its output is taken acrossthe emitter resistor 170 and transmitted through a coupling capacitor172 and resistor 174 to an amplifying circuit 176 which may be of anyappropriate type to amplify and fundamental reference frequencycomponent. In the preferred embodiment the amplifier 176 is anintegrated circuit operational amplifier, but it is well recognized thatother amplifying means can well be used. It is because of the use of anoperational amplifier that the circuits therebeyond in FIG. 6 arerelated to the three voltage levels of 23 volts, +12 volts and ground.

The output of the amplifier 176 on conductor 178 is connected to oneinput of the gated AND circuit 60 through resistor 180 and to theinverter circuit 66 through resistor 182. The other input to the gatedAND circuit 60 is received either from the off-pattern control onconductor 130 through switch contacts 18a, conductor 185, diode 181 andresistor 184 in the case of line tracing or from positive voltage sourcein the case of edge tracing. If the voltage on line 185 is high, thecapacitor 186 may be charged in accordance with the am plitude of thesignal on conductor 178 by means of resistor 180 and resistor 188. Whenthe charge on capacitor 186 builds up to a certain level representativeof the absolute amplitude of the fluctuating signal on conductor 178, asiliconcontrolled rectifier which forms the memory cell 70 is gated intoconduction in a well-known manner. The amplitude level of thefluctuating signal on conductor 178 at which the memory cell 70 is gatedon is determined by the values of resistors 180, 188 and determines theBI threshold level. The anode of memory cell SCR 70 is connected to thebase of a normally conducting transistor 190'which is turned offwhenever the SCR 70 conducts. The collector circuit of the transistor190 therefor represents the Q input to the AND gate 68 which will bedescribed more fully hereinafter.

The output of the operational amplifier 176 on conductor 178 is alsoapplied to the base of the inverter transistor 192. This transistor isnormally biased below cutoff. The output of the operational amplifier onconductor 178 is approximately midway between ground and the sourcevoltage when there is no 60-cycle signal applied thereto which is thevoltage appearing at the cathode of zener diode 195 and hence theemitter of transistor 192. Thus, the voltage dividing effect ofresistors 182, 194 provides a reverse biasing voltage at the base of thetransistor 192, Whenever the operational amplifier has a fluctuatingsignal at its output with an amplitude sufficient to overcome thereverse bias, the transistor 192 will conduct during each portion of thepositive swings'in excess of the reverse bias. The sensing signal levelrequired to overcome the reverse bias transistor 192 establishes the B,threshold level of FIGS.

2b and 312. When .the transistor 192 conducts it provides a signifyingthe high A function and that transistor 190 (FIG. 6)

is not conducting indicating the high Qfunction. There will thus appearon conductor 206 leading to the base of transistor .208 a signal havingthe characteristics indicated by the waveform 210. That is, as long as aline is being viewed as indicated by a high A function, the B1 thresholdlevel has been previously reached as indicated bya high function, andthe signal level on conductor 178 has an amplitude level great enough totrigger transistor 192 into conduction, a sawtooth waveform wiil appearon conductor 206 leading to the base of transistor 208. The timeconstant of the charging circuits for capacitor 198 is selected so thatthe peak voltage of the sawtooth never reaches the source voltagebetween the periodic conduction states of transistor 192. The circuitsof transistor 208 are arranged so that it will not be triggered intoconduction whenever the sawtooth signal is present at its base but willbe triggered into conduction by the steady source voltage. The zenerdiode 212 and resistor 214 in the emitter circuit connection to the baseof the flip-flop transistor 146 govern the level at which transistor 208will conduct and are selected such that it will turn on at a levelbetween'the maximum peak of the sawtooth waveform and the source voltagelevel. Thus, transistor 208 conducts only when the signal level onconductor 178 drops below the B threshold level and the conduction oftransistor 208 then provides the reset signal to the flip-flop 58.

In summary, the inverter circuit 66 and AND circuit 68 cooperate tomaintain the flip-flop" 58 in its set state as established by theinitial operation of button 56 until the tracer arrives at thepredeterminedposition closely adjacent the pattern contour. For example,in line tracing when the sensing head is not viewing the line, both thefunction and the A function are low and the transistor 208 is held offby the low voltage on conductor 206. When the sensing head starts tosense the line the signal amplitude begins to increase and eventuallysurpasses the B level at which the reverse bias of transistor 192 isovercome. Transistor 192 doesnt conduct at this time, however, becauseof its low collector voltage until both the off-line threshold (Afunction) and the B1 threshold (Q function) are reached. When the tracergets close enough to the line to satisfy the A and 0 functions,transistor 192 can conduct and the sawtooth signal appears on conductor206. Transistor 208 is still not able to conduct and remains off untilthe 60 hertz signal level falls below the 8, level at which transistor192 can no longer conduct. Since the Q and A functions are both high atthis time, the capacitor 198 can charge uninterrupted toward the sourcevoltage and will reach the level at which transistor 208 is forwardbiased into conduction. The flip-flop 58 is then reset and opens thegate 92 in the steering circuit to permit the normal operation of thesystem to follow along the pattern line.

The system works similarly in the edge-tracing mode, but since there isno ambiguous preliminary situation, the off-pattern threshold (Afunction) and B1 threshold (0 function) are not significant to theoperation. When the system is being used as an edge tracer the switchcontacts 18b are therefore connected to the source voltage rather thanto the off-pattern circuit in FIG 5. The Q function is establishedimmediately when the tracer is started toward the edge from a positionofi' of the edge because of the maximum amplitude-sensing head signal onconductor 178 at that point which is ended with the source voltage onconductor through diode 182 and resistor 184. SCR 70 is turned on whichturns transistor off to provide a high Q signal to the collector circuitof transistor 192. Since the 60 hertz sensing head signal is greaterthan the B, level at that time, the sawtooth signal immediately appearsat conductor 206 to maintain transistor 208 nonconducting. When thepredetermined position close to the edge is reached, the signalamplitude falls below the B threshold. Transistor 192 is cut offcompletely permitting capacitor 198 to charge up and trigger transistor208 into conduction. The flip-flop resets and the tracer smoothlycatches the edge and follows it in the normal manner. 1

While there has been described-herein a preferred embodiment of theinvention, it is to be understood that modifications and additions maybe made thereto without materially deviating from the teachings of theinvention. It is therefore intended to be bound only by the scope of theappended claims.

We claim:

1. A pattern contour-tracing system comprising a rotatable patternsensing head having a front-to-back axis, means for driving the sensingheadin the direction of the front-to-back axis, means for automaticallyrotating the sensing head responsive to deviations of the position 'ofsaid sensing head from said contour to cause said sensing head to followsaid pattern contour, control means for causing the sensing head tocatch the contour from a position off of the contour, said control meanscomprising means for pointing the front-to-back axis toward the contour,means including said drive means for causing said sensing head toapproach the pattern, means for detecting when the sensing head reachesa position a predetermined distance from the contour, and meansresponsive to said detecting means for preventing the operation of saidautomatic rotating means until the sensing head reaches saidpredetermined distance position.

2. In the pattern contour-tracing system of claim 1 wherein said sensinghead comprises sensing means for producing an electrical signal theamplitude of which is indicative of the transverse deviation of saidsensing head from the contour, and wherein said detecting means operatesresponsive to said electrical signal.

3. In the system of claim 2 wherein said sensing means are adapted toproduce an electrical signal the amplitude of which approaches a firstpredetermined level as the sensing head approaches the pattern contour,and wherein said detecting means causes said preventing means tomaintain said automatic rotating means inoperable until the amplitude ofsaid electrical signal is within a predetermined range of said firstpredetermined level, the amplitude level of the outer limit of saidpredetermined range being representative of said predetermined distance.

4. in the system of claim 3 wherein said detecting means comprises meansoperable responsive only to an electric signal which reaches said-outerlimit amplitude level from a level outside of said predetennined rangeto disenable said preventing means to activate said rotating means.

5. In the system of claim 4 wherein said disenabling means comprises amemory cell operable responsive only to an electric signal amplitudelevel outside of said predetermined range and a gate operable responsiveto the operation of said memory cell and the subsequent attainment ofsaid outer limit amplitude level the output of which controls saidpreventing means.

6. In the system of claim 2 wherein said sensing means is adapted toproduce an electric signal the absolute amplitude of which drops towarda minimum as the sensing head approaches the contour, and wherein saiddetecting means causes said preventing means to maintain said automaticrotating means inoperable until the amplitude of said electrical signaldrops below a predetermined amplitude level above said minimumrepresentative of said predetennined distance.

7. In the system of claim 6 wherein said detecting means comprises meansoperable responsive only to a decreasing absolute amplitude signal tosaid predetermined level for dis enabling said preventing means toactivate said rotating means.

8. in the system of claim 7 wherein said disenabling means comprises amemory cell operable responsive only to an absolute amplitude signalgreater than said predetermined level and a gate operable responsive tothe operation of said memory cell and the subsequent attainment of saidpredetermined absolute amplitude level signal the output of whichcontrols said preventing means.

9. A pattern contour-tracing system comprising a sensing head rotatableabout an axis and having a front-to-back axis, means for driving saidhead in the direction of said front-toback axis, photosensitive means insaid head, means for projecting an image of a portion of said patternforward of said rotational axis on said photosensitive means to causesame to produce electrical signals indicative of deviations of saidphotosensitive means from the image of said pattern contour, means forautomatically rotating said sensing head toward said contour responsiveto said electrical signals to cause same to follow said contour, andcontrol means for causing the sensing head to automatically catch andfollow said contour from a position off of said contour, said controlmeans comprising means for pointing the front-to-back axis toward thepattern contour, means including said driving means for causing saidsensing head to approach the pattern, means including saidphotosensitive means for detecting when the sensing head reaches aposition a predetermined distance from the contour, and means responsiveto said detecting means for preventing the operation of said automaticrotating means until the sensing head reaches said predetermineddistance position.

10. In the system of claim 9 wherein said photosensitive means areadapted to produce an electric signal the amplitude of which approachesa first predetermined level as the sensing head approaches the patterncontour, and wherein said detecting means causes said preventing meansto maintain said automatic rotating means inoperable until the amplitudeof said electric signal is within a predetermined range of said firstpredetermined level, the amplitude level of the outer limits of saidpredetermined range being representative of said predetermined distance.

11. in the system of claim 10 wherein said detecting means comprisesmeans operable responsive only to an electric signal which reaches saidouter limit amplitude level from a level outside of said predeterminedrange for disenabling said preventing means to activate said rotatingmeans.

12. in the system of claim 11 wherein said disenabling ineans comprisesa memory cell operable responsive only to an electric signal amplitudelevel outside of said predetermined range and a gate operable responsiveto the operation of said memory cell and the subsequent attainment ofsaid outer limit amplitude level, the output of which controls saidpreventing means.

13. In the system of claim '9 wherein said photosensitive means isadapted to produce an electric signal the absolute amplitude of whichdrops toward a minimum as the sensing head approaches the contour, andwherein said detecting means causes said preventing means to maintainsaid automatic rotating means inoperable until the amplitude of saidelectrical signal drops below a predetermined amplitude level above saidminimum representative of said predetermined distance.

14. In the system of claim 13 wherein said detecting means comprisesmeans operable responsive only to a decreasing absolute amplitude signalto said predetermined level for disenabling said preventing means toactivate said rotating means.

15. in the system of claim 14 wherein said disenabling means comprises amemory cell operable responsive only to an absolute amplitude signalgreater than said predetermined level and a gate operable responsive tothe operation of said memory cell and the subsequent attainment of saidpredetermined absolute amplitude level signal the output of whichcontrols said preventing means.

16. In the system of claim 9 in which said pattern contour is a line andwherein said photosensitive means is adapted to scan the image of saidpattern across a centerline of scan corresponding to said front-to-backaxis at a reference frequency to produce a fluctuating signal, saidelectrical signal being a reference frequency component of saidfluctuating signal which is present whenever an image of said pattern isbeing scanned except when the centerline of scan is aligned with theimage of said pattern line.

17. In the system of claim 13 in which said pattern contour is a lineand wherein said photosensitive means is adapted to scan the image ofsaid pattern across a centerline of scan corresponding to saidfront-to-back axis at a reference frequency to produce a fluctuatingsignal, said electrical signal being a reference frequency component ofsaid fluctuating signal which is present whenever an image of saidpattern is being scanned except when the centerline of scan is alignedwith the I image of said pattern line.

18. In the system of claim 15 in which said pattern contour is a lineand wherein said photosensitive means is adapted to scan the image ofsaid pattern across a centerline of scan corresponding to saidfront-to-back axis at a reference frequency to produce a fluctuatingsignal, said electrical signal being a reference frequency component ofsaid fluctuating signal which is present whenever an image of saidpattern is being scanned except when the centerline of scan is alignedwith the image of said pattern line.

19. In the system of claim 9 in which said pattern contour is an edgeformed on the pattern between two contrasting colors, and wherein saidphotosensitive means is adapted to scan said edge at a referencefrequency to produce a fluctuating signal, said electrical signal beinga reference frequency component of said fluctuating signal which ispresent when said photosensitive means is not aligned with said edge andis absent when said photosensitive means is aligned with said edge.

20. In the system of claim 13 in which said pattern contour is an edgeformed on the pattern between two contrasting colors, and wherein saidphotosensitive means is adapted to scan said edge at a referencefrequency to produce a fluctuating signal, said electrical signal beinga reference frequency component of said fluctuating signal which ispresent when said photosensitive means is not aligned with said edge andis a absent when said photosensitive means is aligned with said edge.

21. in the system of claim 15 in which said pattern contour is an edgeformed on the pattern between two contrasting colors, and wherein saidphotosensitive means is adapted to scan said edge at a referencefrequency to produce a fluctuating signal, said electrical signal beinga reference frequency component of said fluctuating signal which ispresent when said photosensitive means is not aligned with said edge andis absent when said photosensitive means is aligned with said edge.

1. A pattern contour-tracing system comprising a rotatable patternsensing head having a front-to-back axis, means for driving the sensinghead in the direction of the front-to-back axis, means for automaticallyrotating the sensing head responsive to deviations of the position ofsaid sensing head from said contour to cause said sensing head to followsaid pattern contour, control means for causing the sensing head tocatch the contour from a position off of the contour, said control meanscomprising means for pointing the front-to-back axis toward the contour,means including said drive means for causing said sensing head toapproach the pattern, means for detecting when the sensing head reachesa position a predetermined distance from the contour, and meansresponsive to said detecting means for preventing the operation of saidautomatic rotating means until the sensing head reaches saidpredetermined distance position.
 2. In the pattern contour-tracingsystem of claim 1 wherein said sensing head comprises sensing means forproducing an electrical signal the amplitude of which is indicative ofthe transverse deviation of said sensing head from the contour, andwherein said detecting means operates responsive to said electricalsignal.
 3. In the system of claim 2 wherein said sensing means areadapted to produce an electrical signal the amplitude of whichapproaches a first predetermined level as the sensing head approachesthe pattern contour, and wherein said detecting means causes saidpreventing means to maintain said automatic rotating means inoperableuntil the amplitude of said electrical signal is within a predeterminedrange of said first predetermined level, the amplitude level of theouter limit of said predetermined range being representative of saidpredetermined distance.
 4. In the system of claim 3 wherein saiddetecting means comprises means operable responsive only to an electricsignal which reaches said outer limit amplitude level from a leveloutside of said predetermined range to disenable said preventing meansto activate said rotating means.
 5. In the system of claim 4 whereinsaid disenabling means comprises a memory cell operable responsive onlyto an electric signal amplitude level outside of said predeterminedrange and a gate operable responsive to the operation of said memorycell and the subsequent attainment of said outer limit amplitude levelthe output of which controls said preventing means.
 6. In the system ofclaim 2 wherein said sensing means is adapted to produce an electricsignal the absolute amplitude of which drops toward a minimum as thesensing head approaches the contour, and wherein said detecting meanscauses said preventing means to maintain said automatic rotating meansinoperable until the amplitude of said electrical signal drops below apredetermined amplitude level above said minimum representative of saidpredetermined distance.
 7. In the system of claim 6 wherein saiddetecting means comprises means operable responsive only to a decreasingabsolute amplitude signal to said predetermined level for disenablingsaid preventing means to activate said rotating means.
 8. In the systemof claim 7 wherein said disenabling means comprises a memory celloperable responsive only to an absolute amplitude signal greater thansaid predetermined level and a gate operable responsive to the operationof said memory cell and the subsequent attainment of said predeterminedabsolute amplitude level signal the output of which controls saidpreventing means.
 9. A pattern contour-tracing system comprising asensing head rotatable about an axis and having a front-to-back axis,means for driving said head in the direction of said front-to-back axis,photosensitive means in said head, means for projecting an image of aportion of said pAttern forward of said rotational axis on saidphotosensitive means to cause same to produce electrical signalsindicative of deviations of said photosensitive means from the image ofsaid pattern contour, means for automatically rotating said sensing headtoward said contour responsive to said electrical signals to cause sameto follow said contour, and control means for causing the sensing headto automatically catch and follow said contour from a position off ofsaid contour, said control means comprising means for pointing thefront-to-back axis toward the pattern contour, means including saiddriving means for causing said sensing head to approach the pattern,means including said photosensitive means for detecting when the sensinghead reaches a position a predetermined distance from the contour, andmeans responsive to said detecting means for preventing the operation ofsaid automatic rotating means until the sensing head reaches saidpredetermined distance position.
 10. In the system of claim 9 whereinsaid photosensitive means are adapted to produce an electric signal theamplitude of which approaches a first predetermined level as the sensinghead approaches the pattern contour, and wherein said detecting meanscauses said preventing means to maintain said automatic rotating meansinoperable until the amplitude of said electric signal is within apredetermined range of said first predetermined level, the amplitudelevel of the outer limits of said predetermined range beingrepresentative of said predetermined distance.
 11. In the system ofclaim 10 wherein said detecting means comprises means operableresponsive only to an electric signal which reaches said outer limitamplitude level from a level outside of said predetermined range fordisenabling said preventing means to activate said rotating means. 12.In the system of claim 11 wherein said disenabling means comprises amemory cell operable responsive only to an electric signal amplitudelevel outside of said predetermined range and a gate operable responsiveto the operation of said memory cell and the subsequent attainment ofsaid outer limit amplitude level, the output of which controls saidpreventing means.
 13. In the system of claim 9 wherein saidphotosensitive means is adapted to produce an electric signal theabsolute amplitude of which drops toward a minimum as the sensing headapproaches the contour, and wherein said detecting means causes saidpreventing means to maintain said automatic rotating means inoperableuntil the amplitude of said electrical signal drops below apredetermined amplitude level above said minimum representative of saidpredetermined distance.
 14. In the system of claim 13 wherein saiddetecting means comprises means operable responsive only to a decreasingabsolute amplitude signal to said predetermined level for disenablingsaid preventing means to activate said rotating means.
 15. In the systemof claim 14 wherein said disenabling means comprises a memory celloperable responsive only to an absolute amplitude signal greater thansaid predetermined level and a gate operable responsive to the operationof said memory cell and the subsequent attainment of said predeterminedabsolute amplitude level signal the output of which controls saidpreventing means.
 16. In the system of claim 9 in which said patterncontour is a line and wherein said photosensitive means is adapted toscan the image of said pattern across a centerline of scan correspondingto said front-to-back axis at a reference frequency to produce afluctuating signal, said electrical signal being a reference frequencycomponent of said fluctuating signal which is present whenever an imageof said pattern is being scanned except when the centerline of scan isaligned with the image of said pattern line.
 17. In the system of claim13 in which said pattern contour is a line and wherein saidphotosensitive means is adapted to scan the image of said pattern acrossa centerline of scan corresponding to sAid front-to-back axis at areference frequency to produce a fluctuating signal, said electricalsignal being a reference frequency component of said fluctuating signalwhich is present whenever an image of said pattern is being scannedexcept when the centerline of scan is aligned with the image of saidpattern line.
 18. In the system of claim 15 in which said patterncontour is a line and wherein said photosensitive means is adapted toscan the image of said pattern across a centerline of scan correspondingto said front-to-back axis at a reference frequency to produce afluctuating signal, said electrical signal being a reference frequencycomponent of said fluctuating signal which is present whenever an imageof said pattern is being scanned except when the centerline of scan isaligned with the image of said pattern line.
 19. In the system of claim9 in which said pattern contour is an edge formed on the pattern betweentwo contrasting colors, and wherein said photosensitive means is adaptedto scan said edge at a reference frequency to produce a fluctuatingsignal, said electrical signal being a reference frequency component ofsaid fluctuating signal which is present when said photosensitive meansis not aligned with said edge and is absent when said photosensitivemeans is aligned with said edge.
 20. In the system of claim 13 in whichsaid pattern contour is an edge formed on the pattern between twocontrasting colors, and wherein said photosensitive means is adapted toscan said edge at a reference frequency to produce a fluctuating signal,said electrical signal being a reference frequency component of saidfluctuating signal which is present when said photosensitive means isnot aligned with said edge and is a absent when said photosensitivemeans is aligned with said edge.
 21. In the system of claim 15 in whichsaid pattern contour is an edge formed on the pattern between twocontrasting colors, and wherein said photosensitive means is adapted toscan said edge at a reference frequency to produce a fluctuating signal,said electrical signal being a reference frequency component of saidfluctuating signal which is present when said photosensitive means isnot aligned with said edge and is absent when said photosensitive meansis aligned with said edge.